I have a reproduction hood tach installed on my GSX as the original is beat. I plugged into The hood tach harness from the dash harness. The strange thing is it does not work unless the ligths are on??? There are three wires to the hood tach. A brown wire that goes to the dah harness and must do a Uturn back to the neg side of the coil. The black wire is the ground and the gray is for the tach lights. The lights do work as well. The funny thing is that when my multi meter tach wire is probed into the brown wire connection at the tach I get a signal all the time even though the hood tach is not working? Maybe it is grounding itself through the gray wire? All grounds appear to be good except the strap between the dash clock and the speedo.
Dave I have the same problem. I bought the tach over 20 years ago and just put it on 3 years ago. I forget where I bought it from , maybe Year-One. I am going to use a Ignition on power to the light wire. I believe the tach need a power on and it uses the light wire. I haven't found the time to check this out but first chance I have I will. I will let you know how I make out.
I don't have experience with the GSX hood tach, but all of the aftermarket and diagnostic tachs I have ever worked with needed +12V to operate, in addition to the signal wire, the ground, and the +12V for the lights (if applicable). The instructions on the MFS Performance website (I believe they make most of the repros out there?) say that the tach should have 3 wires- one black for ground, another black for +12V, and a brown for the coil signal. I think the light is just always on, which is sort of silly. The tachometer is basically a three-legged transistor. +12V is present at the input leg. When a voltage is present at the signal leg (aka when the breakers in the distributor are closed), the transistor allows current to flow between the input leg and the output leg. The current flows from the output leg to energize the mechanism that moves the needle, and then goes to ground. The more frequently the tach is energized, the farther the needle climbs, giving a good estimation of engine speed.
Thanks Guys, Looks like I need to run another wire. Not good on a corrrect car. Looks like I will have to find an original The damn thing gets wet inside when you wash it as well. The vent system was not available in 70.
It is odd. I was just reading over in the other thread, and yours is definitely wired the way the factory instructions say to... I guess the factory one must actually use the +V from the trigger wire to operate the tach, while the reproduction needs a separate +12V source. Call MFS Performance maybe? 812.422.5761
I spoke with MFS and they do not make the tachs but know a little about them. The guy I spoke with said the tach needs power unlike the stock tach and the goofy non stock harness needs to be usd for it to work. Why in the hell can't they make it work with the stock harness when even the connector is the same and plugs right into stock harness???
Dave, When I used a repro hood tach for the X-Camino I the brought the wires thru the firewall, wired it up as required, and then brought correct looking wires out thru the firewall. This way it looked correct under the hood. Then I wire tied the "mess" up under the dash and put the rest of the car together. KISS (Keep it simple stupid.) If you don't like how its made send your old one out and get it fixed. If that's even possible today. Duane
You can send tachs (OEM and repro) to me and I can repair them for very low cost. I can't do silkscreening or printing but can paint the needle if required.
Guys, The stock tach I have needs a lot of love. The dial has rust as well. I may just use the aftermarket as is for now and run the parking lights on. Looking for a good rebuildable stock tach. Have one lead so hope it works out. May try and fix the wiring as Duane mentioned as a last resort.
I'm sure its just a money thing. Whoever builds these put their time and money into the presentation, but went with a generic mechanism for the actual tachometer to cut costs. I am sure 90% of the aftermarket hood tachs they sell go on clones and customs, where nobody really cares about adding wiring.
Dave, I had the same problem with my reproduction hood tach. I fixed it by slightly re-wiring the hood tach harness in the dash. Just jump a 12 volt wire to the gray wire in the hood tach harness so as soon as you turn the key on, the hood tach light comes on and the tach works without having to turn the lights on. Yes, the hood tach is lit up all the time when the car is running, but you can't see that the light is on during the day. A very simple fix and no extra wires.
James, did you have to splice the 12 volt wire into the tach harness gray wire somewhere? I am having a little trouble figuring out how you would do that without cutting into the harness somewhere. The hood tach extension wire comes out of the main harness then goes through the firewall and plugs into the tach.
Dave, On factory hood tach cars, there is a three wire extension harness that runs from the hood tach thru the firewall (along with the single brown wire from the coil) between the fuse block and emergency brake. At the end of the three wire extension harness is where I removed one of the wires in the plug on the extension harness and used a 12 volt wire and connected it to the hood tach grey wire. It was 6 years ago and I'm having a hard time remembering, but I may have used one of the two unused pink wires that are close to the wires coming out of the main dash harness where the clock is. You will need to remove the radio to access this area to hopefully prevent removing the dash pad and pulling the dash bezel out. Hope that helps, and makes a little sense. Feel free to send me your number and I can probably make it easier on the phone.
James, Thanks I think I get it now. I was thinking of removing that gray wire frome the connector and putting a factory style connecotr on a twelve volt wire. I may go to the fuse box for power.
